This invention relates to armatures for conducting very large currents between parallel rails of electromagnetic launchers and more particularly to such armatures employing multiple conducting fibers to conduct current between the launcher rails.
In the electromagnetic propulsion of projectiles, a very large dc current, on the order of several hundred thousand amperes, is injected into the breech end of a pair of parallel rails. A projectile which is in sliding contact with the rails is driven toward the muzzle end of the rails where it is ejected at a very high velocity, on the order of several kilometers per second, by the electromagnetic forces generated by the very large current. In many of these projectile launching assemblies, the projectile is provided with an armature which conducts the current between the rails. Originally, the projectile itself was a solid conducting body which served as the armature. Subsequent projectiles included a separate armature, made up of "leaves" of conductive material affixed to the rear of the projectile. The leaves were stacked in the direction of movement of the projectile with each leaf bridging the gap between the rails. The purpose of the laminated armature was to provide better sliding electrical contact between the armature and the rails. Electrical contact was enhanced by making the laminations of resilient, conductive sheets bent about an axis transverse to the direction of armature movement in a chevron configuration so that when the armature was placed between the rails, the ends of each leaf trailed toward the breech end of the launcher and were biased against the adjacent rails. In some configurations the center portion of each lamination was perpendicular to the rails with just the ends trailing rearward.
Experience has shown that with these prior art armatures, the current is concentrated in the corners of the armature adjacent the breech end of the rails. The higher current density at these points causes them to become hot spots. While the multiple leaf configuration reduces this current concentration somewhat, true uniform current distribution among multiple leaves cannot be achieved (at any armature velocity) no matter how thin the individual leaves are made if they are all identical. In fact, the discovery that practically all of the current was carried by the rearmost leaf, led one researcher to discard all of the chevron shaped leaves except one. This single leaf was then laminated from sheets oriented perpendicular to the rail faces to provide the multifinger contact between the rails and the armature.
Two of the most critical design parameters for a metallic armature are the contact force and the compliance. The magnitude of the contact force must be sufficient to maintain the contact drop at a low voltage in order to prevent rail damage caused by arcing. The greater the armature current density, the greater the contact force required for a low resistance current junction. The highest contact force achievable is limited by the allowable stress levels in the armature materials.
To maintain steady contact force, the armature must have sufficient compliance to accommodate both its own wear and changes in the distance between the launcher rails. Therefore, the armature must first develop the required contact force and then maintain it during the launch.
According to the invention, an armature for conducting very large currents between a pair of electrically conductive parallel rails while being driven down the rails under the influence of the electromagnetic forces generated by the application of the very large current to the rails, comprises a plurality of conductive fibers to provide multiple conductor rail contact points and to conduct current between the rails. These conductors are attached to an insulating support member. Transposition of the conductive fibers reduces electrical skin effect. The fibers may be attached to the contact surfaces of a chevron to increase structural rigidity.
An electromagnetic launcher employing a multiple fiber armature in accordance with this invention includes a high current power supply which is connected to a pair of projectile launching rails. A multiple fiber armature is slidably disposed between these rails with the multiple conductive fibers providing a path for current between the rails.